The digital television (DTV) signals currently broadcast in the United States of America employ 8-level vestigial-sideband amplitude-modulation of a suppressed carrier 310 kilohertz above the lower boundary frequency of a 6-megahertz-wide television channel allocation. A fixed-amplitude pilot carrier at the frequency and phase of the suppressed carrier accompanies this 8-VSB signal. The 8-VSB signal is demodulated in the digital television receiver to recover an 8-level baseband DTV signal subjected to data slicing procedures during symbol decoding. Linearity of the DTV signal during its transmission and reception must be preserved, so that the data slicing procedures have low enough rate of errors that the errors are nearly always correctable in reliance upon forward-error-correction coding. The 8-VSB signal has a 10.76 million symbols per second baud rate. Therefore, good echo-suppression is also necessary for low error rate when data-slicing the baseband DTV signal.
Equalization of the transmission channel and suppression of echoes is a matter of considerably more importance in DTV signal reception than in analog television signal reception. Over-the-air transmissions are also affected by multipath reception in which signals are received over more than one path, which paths tend to differ in length and thus give rise to effects similar to echoes in a transmission line. Accordingly, these multipath effects are coin in only termed “echoes”. Usually the strongest signal is considered to be a principal signal. A signal arriving before that principal signal is termed a “pre-echo” or “pre-ghost”, and a signal arriving after that principal signal is termed a “post-echo” or “post-ghost”. While multipath reception causes ghost images on the television viewscreen during analog TV signal reception, complete loss of viewable picture rarely occurs and that loss is usually due to loss of display synchronization or excessive noisiness of the picture owing to weak-signal reception. The audio signal accompanying an analog TV video signal is acceptable so long as there is viewable picture and often persists even when viewable picture is lost. Multipath or other echo-producing conditions can cause simultaneous complete loss of picture and sound during DTV signal reception. Such loss occurs whenever the error rate of data slicing the baseband DTV signal is too great to be correctable in reliance upon forward-error-correction coding. Adaptive digital filtering for channel-equalization and echo-suppression is usually employed prior to data slicing the baseband DTV signal. This adaptive filtering suppresses the pre-echoes and post-echoes accompanying the principal signal, but exacts a penalty in carrier-to-noise ratio (C/N). If C/N becomes too small, data slicing of the baseband DTV signal exhibits excessive error because the noise is large enough to cause errors in distinguishing between data modulation levels.
DTV signal reception is generally considerably better when an in-doors DTV receiver uses an outdoor antenna, rather than an indoor antenna. Losses in field strength owing to attenuation in building materials are avoided by using the outdoor antenna, so carrier strength is boosted vis-à-vis the internal noise of the receiver to improve C/N. Also, changes in field strength and multipath conditions may occur with indoor antenna reception owing to people and pets moving around in the antenna field. Highly directive outdoor antennas, such as yagis, can reduce the strength of multipath vis-à-vis the principal component of received signal, reducing the need for extensive channel-equalization and its attendant penalty in carrier-to-noise ratio (C/N).
The outdoor antenna is coupled to the indoor receiver via a download, which downlead exhibits a transmission loss that is often significant. A common practice to offset these transmission losses, and to boost signal strength in weak signal-strength reception areas, is the installation of a wide-band radio-frequency (RF) amplifier close to the antenna. This wide-band amplifier amplifies the radio-frequency signals supplied from the antenna via a balun and drives a coaxialable downlead to the indoor receiver. This practice is one taken over from analog television signal reception practice.
When a wide-band RF amplifier close to the antenna is used for driving a coaxial-cable downlead to an indoor receiver, care must be taken to avoid strong antenna response at certain wavelengths driving the amplifier into non-linear operation. This overload condition is not only deleterious to the linearity of a strong DTV signal, but causes cross-modulation with any accompanying weaker DTV signal, adversely affecting its linearity as well. These losses of linearity will raise the rate of errors in data slicing the baseband DTV signal sufficiently that the errors no longer are correctable in reliance upon forward-error-correction coding. Filtering to attenuate a strong-signal channel can be introduced into the coupling between the antenna and the wide-band RF amplifier, but the design of the filtering becomes onerous when more than one strong-signal channel must be attenuated. Generally, a TV service technician in the field will be unable to solve these reception problems optimally.
Over-the-air terrestrial television broadcasting is done in the United States of America using 6 MHz wide channels located in three discrete frequency bands, TV broadcast channels 2 through 6 are in a lower VHF band extending from 54 to 88 MHz. TV broadcast channels 7 through 13 are in an upper VHF band extending from 174 to 216 MHz. TV broadcast channels 14 through 83 are in a UHF band extending from 470 to 890 MHz, but some of these uppermost UHF channels will no longer be available for TV broadcasting. When a wide-band RF amplifier close to the antenna is used for driving a coaxial-cable downlead to an indoor receiver, there is difficulty in terminating this transmission line with its characteristic impedance for all of the TV broadcast channels. The coaxial-cable downlead appears to be an infinite-length transmission line for DTV signals broadcast at frequencies for which the coaxial cable is terminated with its characteristic impedance. So, there will be no reflections in the downlead when receiving such a DTV signal TV signals broadcast at signal frequencies at which this transmission line is not terminated with its characteristic impedance will be subject to echoes caused by reflections in the downlead, however. The downlead will often be a source of echoes for some of the DTV signals received by the DTV receiver with outdoor antenna, whether a wide-band RF amplifier is used to drive the downlead, or whether the downlead connects directly from the antenna. The design of a tuner that exhibits characteristic impedance at its RF input connection for every channel in each of the three discrete frequency ranges is extremely difficult, even when different RF stages are used for the UHF and VHF bands.
The inventor points out that thinking about television system design is influenced by TV receiver design of the distant past. Originally, rather sizable electro-mechanical devices were used for channel selection, which devices were operated manually by the human viewing and listening to the TV receiver. These devices are not well suited for inclusion within a remote tuner that is located nearby an outdoor antenna or incorporated into the structure of the antenna. When remote-control devices for tuning TV receivers became commonplace, electrically controlled tuning displaced electro-mechanical devices for channel selection. In recent years monolithic integrated circuitry and surface-acoustic-wave (SAW) filters have virtually eliminated the need for servicing the low-power electronics portions of TV receivers; there is no longer need for replacing vacuum tubes or re-tuning tuned circuitry that has drifted from correct tuning. The monolithic integrated circuitry and SAW filters have reduced the size of the front-end section of a TV receiver, up to and including the intermediate-frequency (IF) amplifier.
The inventor points out that the improvement in reliability and reduction in size of this front-end section of the TV receiver makes feasible a remote tuner located nearby an outdoor antenna or incorporated into the structure of the antenna. This remote tuner is designed to drive a coaxial-cable downlead with intermediate-frequency (IF) signal. In order to eliminate reflections of the IF signal, the coaxial cable is terminated with its characteristic impedance in the IF signal frequency range. Since any TV channel the remote tuner selects for reception is converted to repose in the same 6 MHz wide IF channel, the input coupling network required to terminate the coaxial cable in its characteristic impedance is the same, no matter which TV channel is selected for reception. This eliminates need for re-tuning, the input coupling network in order to terminate the coaxial cable in its characteristic impedance when different DTV broadcast channels are selected for reception.
Preferably, reflex methods are employed to carry up operating power and remote-control signals to the remote tuner via the coaxial-cable downlead. Alternatively, operating power can be conducted to the remote tuner via separate connection. The remote-control signals for the remote tuner can be conducted to it via separate connection. Modulation of a carrier with the remote-control signals facilitates the remote-control signals being, conducted to the remote tuner via the coaxial-cable downlead by frequency multiplexing.
The remote tuner is more easily designed to avoid being overloaded by strong signals than a wide-band RF amplifier is. The remote-control signals for the remote tuner can be used to control electric tuning of input coupling to the RF amplifier input stage of the remote tuner, as well as to control output coupling of that stage to the following mixer and to control the frequency of local oscillations applied to the mixer. The selectivity of the tuned input coupling to the RF amplifier input stage will reject strong signals not selected for reception, particularly those strong signals in channels more remote from that channel selected for reception.
The remote tuner is preferably designed to include an envelope detector following its final IF voltage amplifier stage. Peaks of the envelope are detected to develop an automatic gain control (AGC) signal applied to the IF voltage amplifier stages. A delayed AGC signal is developed for application to the RF amplifier input stage of the remote tuner, so that a strong signal selected for reception will be prevented from overloading that stage and driving it into non-linear operation.